Class types are the class-level equivalent of type expressions: they
specify the general shape and type properties of classes.

class-type

::=

|

class-body-type

|

[[?]label] typexpr->class-type

class-body-type

::=

object [(typexpr)] {class-field-spec} end

|

class-path

|

[typexpr {,typexpr} ]class-path

class-field-spec

::=

inheritclass-type

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val [mutable] inst-var-name:typexpr

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method [private] method-name:poly-typexpr

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method [private] virtualmethod-name:poly-typexpr

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constrainttypexpr=typexpr

Simple class expressions

The expression class-path is equivalent to the class type bound to
the name class-path. Similarly, the expression
[typexpr1, ... typexprn]class-path is equivalent to
the parametric class type bound to the name class-path, in which
type parameters have been instanciated to respectively typexpr1,
...typexprn.

Class function type

The class type expression typexpr->class-type is the type of
class functions (functions from values to classes) that take as
argument a value of type typexpr and return as result a class of
type class-type.

Class body type

The class type expression
object [(typexpr)] {class-field-spec} end
is the type of a class body. It specifies its instance variables and
methods. In this type, typexpr is matched against the self type, therefore
providing a binding for the self type.

A class body will match a class body type if it provides definitions
for all the components specified in the class type, and these
definitions meet the type requirements given in the class type.
Furthermore, all methods either virtual or public present in the class
body must also be present in the class type (on the other hand, some
instance variables and concrete private methods may be omitted). A
virtual method will match a concrete method, which makes it possible
to forget its implementation. An immutable instance variable will match a
mutable instance variable.

Inheritance

The inheritance construct inheritclass-type allows to include
methods and instance variables from other classes types.
The instance variable and method types from this class type are added
into the current class type.

Instance variable specification

A specification of an instance variable is written
val [mutable] inst-var-name:typexpr, where inst-var-name
is the name of the instance variable and typexpr its expected type.
The flag mutable indicates whether this instance variable can be
physically modified.

An instance variable specification will hide any previous
specification of an instance variable of the same name.

Method specification

The specification of a method is written
method [private] method-name:poly-typexpr, where
method-name is the name of the method and poly-typexpr its
expected type, possibly polymorphic. The flag private indicates
whether the method can be accessed from outside the class.

The polymorphism may be left implicit in method specifications: any
type variable which is not bound to a class parameter and does not
appear elsewhere inside the class specification will be assumed to be
polymorphic, and made explicit in the resulting method type.
Writing an explicit polymorphic type will disable this behaviour.

Several specification for the same method must have compatible types.
Any non-private specification of a method forces it to be public.

Virtual method specification

Virtual method specification is written method [private]
virtualmethod-name:poly-typexpr, where method-name is the
name of the method and poly-typexpr its expected type.

Constraints on type parameters

The construct constrainttypexpr1=typexpr2 forces the two
type expressions to be equals. This is typically used to specify type
parameters: they can be that way be bound to a specified type
expression.

Class expressions are the class-level equivalent of value expressions:
they evaluate to classes, thus providing implementations for the
specifications expressed in class types.

class-expr

::=

class-path

|

[typexpr {,typexpr} ]class-path

|

(class-expr)

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(class-expr:class-type)

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class-expr {argument}+

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fun {parameter}+->class-expr

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let [rec] let-binding {andlet-binding} inclass-expr

|

object [(pattern [:typexpr] )] { class-field } end

class-field

::=

inheritclass-expr [asvalue-name]

|

val [mutable] inst-var-name [:typexpr] =expr

|

method [private] method-name {pattern} [:typexpr] =expr

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method [private] method-name:poly-typexpr=expr

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method [private] virtualmethod-name:poly-typexpr

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constrainttypexpr=typexpr

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initializerexpr

Simple class expressions

The expression class-path evaluates to the class bound to the name
class-path. Similarly, the expression
[typexpr1, ... typexprn]class-path
evaluates to the parametric class bound to the name class-path,
in which type parameters have been instanciated to respectively
typexpr1, ...typexprn.

The expression (class-expr) evaluates to the same module as
class-expr.

The expression (class-expr:class-type) checks that
class-type match the type of class-expr (that is, that the
implementation class-expr meets the type specification
class-type). The whole expression evaluates to the same class as
class-expr, except that all components not specified in
class-type are hidden and can no longer be accessed.

Class application

Class application is denoted by juxtaposition of (possibly labeled)
expressions. Evaluation works as for expression application.

Class function

The expression fun [[?]label] pattern->class-expr evaluates to a
function from values to classes.
When this function is applied to a value v, this value is
matched against the pattern pattern and the result is the result of
the evaluation of class-expr in the extended environment.

Conversion from functions with default values to functions with
patterns only works identically for class functions as for normal
functions.

The expression

funparameter1 ... parametern->class-expr

is a short form for

funparameter1-> ... funparametern->expr

Local definitions

The let and let rec constructs bind value names locally,
as for the core language expressions.

Class body

The expression
object(pattern [:typexpr] ) { class-field } end denotes
a class body. This is the prototype for an object : it lists the
instance variables and methods of an objet of this class.

A class body is a class value: it is not evaluated at once. Rather,
its components are evaluated each time an object is created.

In a class body, the pattern (pattern [:typexpr] ) is
matched against self, therefore provinding a binding for self and self
type. Self can only be used in method and initializers.

Self type cannot be a closed object type, so that the class remains
extensible.

Inheritance

The inheritance construct inheritclass-expr allows to reuse
methods and instance variables from other classes. The class
expression class-expr must evaluate to a class body. The instance
variables, methods and initializers from this class body are added
into the current class. The addition of a method will override any
previously defined methods of the same name.

An ancestor can be bound by prepending the construct asvalue-name
to the inheritance construct above. value-name is not a true
variable and can only be used to select a method, i.e. in an expression
value-name#method-name. This gives access to the
method method-name as it was defined in the parent class even if it is
redefined in the current class.
The scope of an ancestor binding is limited to the current class.
The ancestor method may be called from a subclass but only indirectly.

Instance variable definition

The definition val [mutable] inst-var-name=expr adds an
instance variable inst-var-name whose initial value is the value of
expression expr. Several variables of the same name can be defined
in the same class.
The flag mutable allows physical modification of this variable by
methods.

An instance variables can only be used in the following methods and
initializers of the class.

Method definition

Method definition is written methodmethod-name=expr. The
definition of a method overrides any previous definition of this
method. The method will be public (that is, not private) if any of
the definition states so.

A private method, methodprivatemethod-name=expr, is a
method that can only be invoked on self (from other methods of the
current class as well as of subclasses of the current class). This
invocation is performed using the expression
value-name#method-name, where value-name is directly bound to
self at the beginning of the class definition. Private methods do
not appear in object types. A method may have both public and private
definitions, but as soon as there is a public one, all subsequent
definitions will be made public.

Methods may have an explicitly polymorphic type, allowing them to be
used polymorphically in programs (even for the same object). The
explicit declaration may be done in one of three ways: (1) by giving an
explicit polymorphic type in the method definition, immediately after
the method name, i.e.method [private] method-name: {'ident}+.typexpr=expr; (2) by a forward declaration of the explicit polymorphic type
through a virtual method definition; (3) by importing such a
declaration through inheritance and/or constraining the type of self.

Some special expressions are available in method bodies for
manipulating instance variables and duplicating self:

expr

::=

...

|

inst-var-name<-expr

|

{< [ inst-var-name=expr { ;inst-var-name=expr } ] >}

The expression inst-var-name<-expr modifies in-place the current
object by replacing the value associated to inst-var-name by the
value of expr. Of course, this instance variable must have been
declared mutable.

The expression
{< [ inst-var-name=expr { ;inst-var-name=expr } ] >}
evaluates to a copy of the current object in which the values of
instance variables inst-var-name1, ..., inst-var-namen have
been replaced by the values of the corresponding expressions expr1,
..., exprn.

Virtual method definition

Method specification is written method [private] virtualmethod-name:poly-typexpr. It specifies whether the method is
public or private, and gives its type. If the method is intended to be
polymorphic, the type should be explicit.

Constraints on type parameters

The construct constrainttypexpr1=typexpr2 forces the two
type expressions to be equals. This is typically used to specify type
parameters: they can be that way be bound to a specified type
expression.

Initializers

A class initializer initializerexpr specifies an expression that
will be evaluated when an object will be created from the class, once
all the instance variables have been initialized.

A class definition classclass-binding { andclass-binding } is
recursive. Each class-binding defines a class-name that can be
used in the whole expression except for inheritance. It can also be
used for inheritance, but only in the definitions that follow its own.

A class binding binds the class name class-name to the value of
expression class-expr. It also binds the class type class-name to
the type of the class, and defines two type abbreviations :
class-name and #class-name. The first one is the type of
objects of this class, while the second is more general as it unifies
with the type of any object belonging to a subclass (see
section 6.4).

Virtual class

A class must be flagged virtual if one of its methods is virtual (that
is, appears in the class type, but is not actually defined).
Objects cannot be created from a virtual class.

Type parameters

The class type parameters correspond to the ones of the class type and
of the two type abbreviations defined by the class binding. They must
be bound to actual types in the class definition using type
constraints. So that the abbreviations are well-formed, type
variables of the inferred type of the class must either be type
parameters or be bound in the constraint clause.

A class type definition classclass-name=class-body-type
defines an abbreviation class-name for the class body type
class-body-type. As for class definitions, two type abbreviations
class-name and #class-name are also defined. The definition can
be parameterized by some type parameters. If any method in the class
type body is virtual, the definition must be flagged virtual.

Two class type definitions match if they have the same type parameters
and the types they expand to match.